1. Field of the Invention
The present invention relates to a signal processing circuit including a memory circuit in which a stored logic state is not lost even after power is turned off, and to an electronic device using the signal processing circuit. Further, the present invention relates to a method of driving a signal processing circuit including a memory circuit in which a stored logic state is not lost even after power is turned off.
2. Description of the Related Art
Signal processing circuits such as central processing units (CPUs) vary in structure depending on the intended use. A signal processing circuit generally has a main memory for storing data or a program and other memory circuits such as a register and a cache memory.
A register has a function of temporarily holding data in order to carry out arithmetic processing or hold a program execution state, for example. In addition, a cache memory is located between an arithmetic circuit and a main memory in order to reduce access to the low-speed main memory and speed up the arithmetic processing.
In a memory circuit in a signal processing circuit, such as a register or a cache memory, data writing and data reading need to be performed at higher speed than in a main memory. For this reason, in general, a flip-flop circuit is used as a register, and a static random access memory (SRAM) is used as a cache memory. That is, as a memory circuit such as a register or a cache memory, a volatile memory circuit in which data is lost after supply of power supply voltage is stopped is used.
Further, in order to suppress power consumption, a method in which supply of power supply voltage to a signal processing circuit is temporarily stopped in a period during which data is not input/output has been suggested (e.g., see Patent Document 1).
In Patent Document 1, a non-volatile memory circuit is located in the periphery of a volatile memory circuit such as a register or a cache memory. Data is temporarily stored in the non-volatile memory circuit while supply of power supply voltage to a signal processing circuit is stopped.
Meanwhile, in recent years, metal oxides having semiconductor characteristics (also referred to as oxide semiconductors) have attracted attention. Oxide semiconductors can be applied to transistors (e.g., see Patent Documents 2 and 3).